Static drain for outside long wire antenna

[KC1LML]

Up until now I've used the old tube radios I collect and restore for DXing. I use a 50' outside wire for my antenna. Static (other than lightning) isn't a problem for tube radios. But I decided I wanted to try one of the new portable radios and ordered a Tecsun PL-880, which should arrive in a couple of days.

I'm aware that the RF front end of solid state radios can be damaged by static. I was able to find schematics for the Tecsun PL-880. I see it has a reversed pair of 1N4148 diodes connected between its telescopic antenna and ground. I suppose that's to protect the front end should someone touch the antenna with a static charge on their body. However, the external antenna jack doesn't have that kind of protection.




I was looking around the web, and it looks like most add-on static drains consist of (4) 1N4148 diodes, a reversed pair consisting of (2) diodes end-to-end. If that is really necessary, it seems like Tecsun would have done it on the built-in antenna. It seems like all that would do is raise the forward voltage drop from .7 to 1.4 volts. I suppose it would also limit the static current by a minuscule amount. But is that really going to make a difference to high voltage static or a spark?



In any case, I'm concerned. Static is definitely a problem at our house. I have to keep a humidifier running in my radio room in the winter.

 

[ka3jjz]

It does seem to work - at least this circuit was popular when the ICF2010s were blowing out their front end FETs.

One thing to keep in mind, as I wrote a long time back, is that in high RF environments, the diodes might start to rectify and cause noise and spurs all over the place. You can substitute things like a fast acting gas discharge element - even a small flashlight bulb will do. You don't really need to use diodes if you have something else that will blow when there's too much juice on the antenna. In fact, I think Alpha Delta used to (maybe still does) have a discharge unit that used gas elements. The late Perry Ferrell of Gilfer Associates used to do this exact same thing - he would take the diode protection out of the NRD receivers he was selling and substitute the Alpha Delta unit...Mike

 

[prcguy]

You can also bypass the antenna lead with a choke that has high impedance within the HF bands but will short out DC or very low frequency RF and static buildup. In most cases about 75uh to 100uh is a good value that will have virtually no effect to reception but will provide a path to ground for other stuff. These values of inductors are commonly used in bias Tees or power injectors for coax where they insert the voltage onto the coax center conductor and they are invisible to reception or high power transmit.

A Harris employee told me years ago that during the Gulf War the static buildup on HF fiberglass whips in the desert from blowing sand was damaging some radio equipment. The fix from Harris was to install an inductor from the whip hot to ground and this apparently cured the problem. I don't know the specific value that Harris used but it is probably within the range I mentioned.

 

[KC1LML]

The lead-in for my 50' outside 14 gauge antenna wire is just a single 28 gauge insulated wire, so that my window can close on it without any air leakage. We seldom get lightning where I live, but if a storm is forecast, I drop the lead-in outside the window and connect it to the ground rod that is there.

I have a 2 meter antenna on my roof with a coax that runs into the house. The coax goes through a Nagoya H-20 lightning arrester that has a replaceable gas discharge tube in it. I think it is similar to the Alpha Delta unit.

I know I should do something more permanent with my MW/SW antenna and use coax with a proper lightning arrester. But at present I'm more concerned with touching the radio's external antenna jack and the lead-in wire inside the house, where static is a constant problem. Sometimes I think I should attach a light-weight chain to my posterior.

I see that Littlefuse has gas arresters with leads on them. Some go down to 90 volts, which is about the same as 1N4148 diodes, except the gas arresters can take repetitive hits. I think I'll see about getting both gas arresters and the chokes that prcguy mentioned.

I do have some 1N4148's for now. It will be interesting to see if they rectify RF signals around here and cause a problem. I don't think they will, as the nearest radio station is 25 miles away and it's low power.

 

[prcguy]

Back to back diodes are common for limiting the amount of RF a circuit will receive but I don't think its the right fix for static electricity. That can reach 5kV or more and might keep the diodes conducting for long periods of time during blowing wind and low humidity. The current will be very low from static but you could easily exceed the breakdown voltage of the diodes. A choke across the antenna lead to ground is a better fix for static and once the static electricity is under control the diodes can be left to do their job limiting RF levels into your receiver.

Is the 2m antenna grounded to code?

The lead-in for my 50' outside 14 gauge antenna wire is just a single 28 gauge insulated wire, so that my window can close on it without any air leakage. We seldom get lightning where I live, but if a storm is forecast, I drop the lead-in outside the window and connect it to the ground rod that is there.

I have a 2 meter antenna on my roof with a coax that runs into the house. The coax goes through a Nagoya H-20 lightning arrester that has a replaceable gas discharge tube in it. I think it is similar to the Alpha Delta unit.

I know I should do something more permanent with my MW/SW antenna and use coax with a proper lightning arrester. But at present I'm more concerned with touching the radio's external antenna jack and the lead-in wire inside the house, where static is a constant problem. Sometimes I think I should attach a light-weight chain to my posterior.

I see that Littlefuse has gas arresters with leads on them. Some go down to 90 volts, which is about the same as 1N4148 diodes, except the gas arresters can take repetitive hits. I think I'll see about getting both gas arresters and the chokes that prcguy mentioned.

I do have some 1N4148's for now. It will be interesting to see if they rectify RF signals around here and cause a problem. I don't think they will, as the nearest radio station is 25 miles away and it's low power.

 

[KC1LML]

You've convinced me about the choke. I'm going to order a couple of them today.

The 8' ground rod for my 2 meter antenna isn't connected to my entrance panel ground which is about 50' away. I know code requires for it to be connected, but in my defense, my antenna ground isn't used for any power grounding. The price for 50' of #4 copper wire has also delayed me doing it correctly.

 

[prcguy]

This should work fine for most HF freqs. I found a box of 25 a few years back for about $10 and have been using them on lots of projects.

Vintage Miller 4632 100uH 400mA R.F. Choke - for sale online | eBay

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You've convinced me about the choke. I'm going to order a couple of them today.

The 8' ground rod for my 2 meter antenna isn't connected to my entrance panel ground which is about 50' away. I know code requires for it to be connected, but in my defense, my antenna ground isn't used for any power grounding. The price for 50' of #4 copper wire has also delayed me doing it correctly.

 

[KC1LML]

This should work fine for most HF freqs. I found a box of 25 a few years back for about $10 and have been using them on lots of projects.

Vintage Miller 4632 100uH 400mA R.F. Choke - for sale online | eBay

Find many great new & used options and get the best deals for Vintage Miller 4632 100uH 400mA R.F. Choke - at the best online prices at eBay! Free shipping for many products!

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Thanks. I ordered (5) J.W. Miller 5250 100uH 2A Chokes a few minutes before your post. I wouldn't think it would matter that the ones I ordered are 2 amp rated. Turns out the seller is over in the next town about 7 miles away but no way to pick them up in person.

 

[prcguy]

My box of 25 are the 2A version and I've made lots of HF Bias Tees with them. A friend recently handed me several bags of 75uh toroidal inductors that will handle a few amps and those would also be a good choice. If you were local I would give you a handful.

Thanks. I ordered (5) J.W. Miller 5250 100uH 2A Chokes a few minutes before your post. I wouldn't think it would matter that the ones I ordered are 2 amp rated. Turns out the seller is over in the next town about 7 miles away but no way to pick them up in person.

 

[ka3jjz]

If someone were to copy the schematic from the sticky thread (below) and substitute the choke (with the parts reference), this could easily be put into the wiki...thanks to prcguy for the idea...

Homebrew - A great first timer project

We've all read the horror stories of how someone forgot to disconnect their antenna and the radio got zapped - or perhaps how static electricity fried some portable's front end Let's face it; there are going to be times when you will forget to disconnect the antenna (and ground it away from the...

forums.radioreference.com

Mike

 

[KC1LML]

My J.W. Miller 5250 100uH 2A Chokes arrived and here's what I ended up with. The back to back 1N4148 diodes are inside the heat shrink. Reception seems just as good with this inline or not. I don't think the diodes are letting any RF noise come through, but the nearest strong stations are in Boston about 70 miles away.

 

[Ubbe]

Just put a 10k resistor over the antenna terminal inside the receiver. It will bleed off any static build ups due to the climate and weather. The gas arrestors and other things you mention are for lightning protection, not static build up. Two 1N4148 back to back will start to conduct at 1,4Vpp which is +7dBm to protect from temporary RF overload problems. I've seen warning notes for receivers that says max +10dBm input power.

If you usually remove antenna coaxes when there's a risk for lightning strikes, then remember to first bleed out the static from the coax before connecting them to a receiver, if the antenna isn't DC grounded between elements. There are cheap 50/75 ohm $2 BNC (network?) loads you could always have connected to loose antenna coax cables to stop them from having any static voltage.

/Ubbe

 

[prcguy]

The random length end fed wire can present a very high impedance at some frequencies and I think a 10k ohm resistor is too low in value. If your going the resistor route I think its better to use something in the 1meg ohm range or slightly higher and to use a high voltage rated resistor. Here is an article from a guy that writes articles for QST magazine and lives a few houses down from my Texas home, although I have never met him. I do admire his vertical antenna whenever I ride my bike up the alley past his house.

http://www.ad5x.com/images/Articles/Static%20Bleed.pdf

Just put a 10k resistor over the antenna terminal inside the receiver. It will bleed off any static build ups due to the climate and weather. The gas arrestors and other things you mention are for lightning protection, not static build up. Two 1N4148 back to back will start to conduct at 1,4Vpp which is +7dBm to protect from temporary RF overload problems. I've seen warning notes for receivers that says max +10dBm input power.

If you usually remove antenna coaxes when there's a risk for lightning strikes, then remember to first bleed out the static from the coax before connecting them to a receiver, if the antenna isn't DC grounded between elements. There are cheap 50/75 ohm $2 BNC (network?) loads you could always have connected to loose antenna coax cables to stop them from having any static voltage.

/Ubbe

 

[KC1LML]

I keep putting off setting up my long wire antenna the right way. Hope I don't regret it. For the time being, I keep the ground wire clipped to the antenna wire (in my room) when I'm not using one of my radios. A high voltage, high impedance resistor sounds like the way to go. I may have to wait until spring to redo my antenna properly.

 

[Ubbe]

The random length end fed wire can present a very high impedance at some frequencies and I think a 10k ohm resistor is too low in value.

But what's the impedance of the receiver circuit where the resistor are placed? Several mega ohm? If it's a lower value then the resistor shouldn't matter to the antenna impedance.

If your going the resistor route I think its better to use something in the 1meg ohm range or slightly higher and to use a high voltage rated resistor.

Why high voltage if the resistor anyways bleeds all the voltage that could build up by the static charging?

/Ubbe

 

[MUTNAV]

The random length end fed wire can present a very high impedance at some frequencies and I think a 10k ohm resistor is too low in value. If your going the resistor route I think its better to use something in the 1meg ohm range or slightly higher and to use a high voltage rated resistor. Here is an article from a guy that writes articles for QST magazine and lives a few houses down from my Texas home, although I have never met him. I do admire his vertical antenna whenever I ride my bike up the alley past his house.

http://www.ad5x.com/images/Articles/Static%20Bleed.pdf

Oh... and if you want, you could run some leads from the resistor in a way that would be easy to measure the voltage, letting you know just how much voltage the wire antenna picks up from the wind.

Some people are interested in things like that.

Thanks
Joel

 

[prcguy]

I would expect an HF receiver high impedance input to be in the 500 to 1k ohm range and not in the megohms. There could be many thousands of volts across the resistor in a low humidity area with wind blown sand or snow. I've had arcs across open PL-259 connectors during storms and that takes well over 5kV to arc that far. The input circuits of a receiver will pull the voltage way down since there is virtually no current involved with static but an unterminated cable will have full potential across it and that's why a high voltage rated resistor is recommended.

There is also a consideration for transmitting where a random wire or whip can be very high impedance resulting in very high voltages on transmit. The guy in the link I provided knows much more than I do on the topic so I will trust his suggestions.

But what's the impedance of the receiver circuit where the resistor are placed? Several mega ohm? If it's a lower value then the resistor shouldn't matter to the antenna impedance.
Why high voltage if the resistor anyways bleeds all the voltage that could build up by the static charging?

/Ubbe

 

[Ubbe]

I would expect an HF receiver high impedance input to be in the 500 to 1k ohm range and not in the megohms.

So 10k would be perfect then?

There could be many thousands of volts across the resistor in a low humidity area with wind blown sand or snow.

Please explain. The static charge would have to be of a higher current than the resistor can bleed off. If you where to have a 1000V static charge from the antenna it would have to force a 100mA constant current. Ampere = Volt/R. 1kV divided by 10k = 0,1A. 1000V x 0,1A = 100Watt.

/Ubbe

 

[prcguy]

1meg ohm would be perfect in most cases. I don't know the current capability of static electricity but its very very low, you would never see 100ma available. A 1meg resistor will easily bleed off static to a safe level. A 1meg ohm resistor is also commonly used in anti static soldering stations on wrist straps to bleed off static without subjecting the user to a dangerous electrocution hazard if they accidentally touch the local AC mains.

Otherwise read the article in my link above.

Edit: Many people need static protection for antennas that also transmit, consider some of those antennas can be several thousand ohms impedance and matched with an antenna tuner right at the antenna. In that case an antenna with an impedance of say 2,500 ohms with a 10k resistor across it will divide 1/4 of the transmit power to the resistor. That would be a disaster. A 1 meg ohm resistor would only get 1/400 of the transmitter power in this case.

So 10k would be perfect then?

Please explain. The static charge would have to be of a higher current than the resistor can bleed off. If you where to have a 1000V static charge from the antenna it would have to force a 100mA constant current. Ampere = Volt/R. 1kV divided by 10k = 0,1A. 1000V x 0,1A = 100Watt.

/Ubbe

 

[StaticDischarge]

For recieving, Spark plug(s) with a .005 gap on any long wire or twin lead (open wire) will take the most concerning static shocks straight to ground easy enough. (Transmitting will require a larger gap).
Think AM radio lightning discharge systems...